Dr Hessayon says . . .know your soil structure

A gardening expert’s guide

Saturday March 27 2010, 12.01am, The Times

Saturday March 27 2010, 12.01am, The Times

Topsoil is the fertile and living part of the soil. It contains almost all of the humus and supports countless bacteria, which change material into plant foods. This layer varies from little more than an inch in chalky soil to several feet in old, well-tended gardens. When digging, this layer should be turned over, not buried under the subsoil.

Subsoil lies under the topsoil. It is lighter in colour, due to lack of humus. When digging, it should not be brought to the surface.

A soil pan is a horizontal layer on or under the soil surface, which prevents the free movement of air and water to the region below. A surface pan is formed by the action of heavy rain on certain soil types